1. Field of the Invention
This invention relates to the preparation of transparent electroconductive articles by the application of tin oxide film forming compositions to glass or ceramic substrates and, in particular, relates to a novel method for the preparation of such tin oxide film forming compositions.
2. Brief Description of the Prior Art
It is well known that transparent, electroconductive films may be deposited upon refractory bases or substrates, such as glass, by heating substrate surfaces to elevated temperatures or maintaining them at elevated temperatures following formation and contacting the hot substrate surfaces with tin oxide film forming compositions. Typical tin oxide film forming compositions are solutions or mixtures containing tin compounds which are readily decomposed due to heat, particularly in an oxidizing environment, to form tin oxide films. Both organic and inorganic tin compounds have been employed to provide a source of tin and such tin oxide film forming compositions. According to the teachings of U.S. Pat. No. 3,107,177 to Saunders and Wagner, film forming compositions may include organic tin compounds, such as dibutyl tin diacetate, along with a souce of fluorine, such as hydrofluoric acid, for improving the electroconductivity of films produced due to the inclusion of fluorine in those films. That patent further discloses that various organic solvents and solvating agents may be used in preparing such film forming compositions.
In a typical method for preparing tin oxide film forming compositions, a tin-containing compound, such as dibutyl tin diacetate, is first placed in a mixing container. To the dibutyl tin diacetate in the mixing container is added a solution or mixture of hydrofluoric acid in an organic solvent, such as methanol. Typically, a 30 percent hydrofluoric acid-methanol solution may be slowly added to the mixing container containing dibutyl tin diacetate with continuous or intermittent stirring. During the blending of these two ingredients, a semi-solid mixture results which has a granular texture with a consistency much like cottage cheese. While these ingredients are in this semi-solid stage, mixing is continued to insure adequate blending. Mixing of such a semi-solid, highly viscous composition requires expenditure of considerable effort. Following the blending of these two ingredients, a quantity of solvating agent, such as triethylamine, is slowly added to the semi-solid mixture in the mixing container and mixing is continued. During this mixing step, the ingredients react exothermically, generating considerable heat, causing the mixture to boil vigorously and at times producing heavy, white fumes and a noticeably heavy foam on the surface of the mixture. Of course, the mixing necessarily takes place in a vented space under a hood in order to maintain safe and healthy conditions for those individuals preparing such a mixture. Typically, more than 10 percent of the ingredients are lost due to vaporization and fuming during this mixing step.
The present invention contemplates a method for improved mixing of ingredients of a tin oxide film forming composition.